1. Field of the Invention
The present invention relates to a method of manufacturing a preform of an optical fiber used in the field of communication and optical devices.
2. Description of the Related Art
A rod-in-tube method is known to the art as a method of manufacturing an optical fiber preform.
In the rod-in-tube method, a glass rod acting as a core is inserted into a glass tube acting as a cladding. Under this condition, the assembly of the glass tube and the glass rod is heated to cause fusion so as to form an integral body. In the conventional method, however, the glass tube and the glass rod tend to be collapsed in the heating step for fusion, with the result that bubbles tend to remain in the manufactured optical fiber preform. It follows that the optical fiber prepared by drawing the defective optical fiber preform becomes brittle and is low in reliability.
Vigorous researches are being made in an attempt to develop a hydrostatic pressing method which can be used in place of the rod-in-tube method. In this case, a porous preform is prepared by forming a porous clad body by the hydrostatic pressing method on the outer surface of a glass rod acting as a core, followed by dehydrating and sintering the porous preform to form a transparent glass clad layer and, thus, to obtain an optical fiber preform. The hydrostatic pressing method is disclosed in, for example, Published Unexamined Japanese Patent Application Nos. 59-19891 and 61-256937.
In the case of employing the hydrostatic pressing method, a rod body acting as a core such as a glass rod made of, for example, a silica-based material, is put in a mold made of an elastic material. Further, a molding material containing a silica-based powder as a main raw material is loaded around the rod body. Under this condition, the mold is pressurized from outside the mold by a liquid pressure so as to form a porous layer on the surface of the glass rod and, thus, to obtain a porous preform consisting of the core glass rod and the porous clad layer. Then, the porous preform is taken out of the mold, followed by applying drying, degreasing, dehydrating and sintering treatments to the porous preform so as to obtain an optical fiber preform.
Where a porous layer is formed on the surface of a glass rod by the hydrostatic pressing method, a compressing load of about 1.5 tons/cm.sup.2 is applied toward the center of the mold for about 1 to 50 minutes. What should be noted is that the compressing load fails to be applied uniformly to the powdery molding material. As a result, the porous layer formed on the glass rod surface is deformed. It follows that the porous layer fails to be concentric with the glass rod. Alternatively, the glass rod is broken. Particularly, the glass rod breakage is increased with decrease in the diameter of the glass rod. What should also be noted is that the impact produced by the breakage of the glass rod causes the formed porous layer to be cracked or split.
In the conventional method of manufacturing an optical fiber preform, a taper finishing treatment is applied to the end portions of the porous layer such that the outer diameter of the porous layer is diminished toward the edges thereof. The taper finishing treatment is intended to perform smoothly the after-treatments of the porous layer such as the sintering treatment and to prevent the end portions of the porous layer from being broken during handling of the preform.
In applying a taper finishing treatment to the porous layer formed on the glass rod core, the porous preform is disposed in a mold in which ring-like tools for forming the end portions, each having a tapered inner circumferential surface, are disposed in portions corresponding to the end portions of the porous preform. Under this condition, a predetermined liquid pressure is applied to the porous layer. In this molding step, a nonuniformity takes place in the loading density of the molding material such as the silica-based powder in the end portions within the mold, or an air fails to be removed sufficiently in the end portions, resulting in failure to obtain a porous preform of a high quality.
The difficulty described above is derived from the behavior of the molding material in the compression molding step. For example, the glass rod receives a compressing loaded in the radial direction of the mold cavity in the molding step, giving rise to slippage in the axial direction of the glass rod. Since the movement of the glass rod is restricted within the mold, the load is applied nonuniformly to the glass rod. As a result, the stress is locally concentrated on the glass rod. For example, the stress is concentrated in the central portion or both end portions in the longitudinal direction of the glass rod, giving rise to the breakage problem described above.
What should also be noted is that the end portions of the mold have a mechanical strength higher than that in the central portion. Naturally, the porous layer formed in the end portion of the mold is less likely to be deformed than the porous layer formed in the central portion of the mold. Further, the molding material is less likely to be moved in the end portion of the mold than in the central portion. It follows that, when the molding material is pressurized within the mold, the molding material is sufficiently pressurized in the central portion of the mold and moved toward the end portion. In the end portion of the mold, however, the porous layer is unlikely to be deformed. Also, the molding material is unlikely to be moved in the end portion of the mold. Under the circumstances, the loading density of the molding material becomes nonuniform, and air removal from within the mold becomes insufficient.
As described above, in the conventional hydrostatic pressing method, a porous layer is formed on the surface of a core rod member by the hydrostatic pressing method, followed by applying a purification and sintering treatment to the resultant porous preform so as to manufacture a preform for an optical fiber. In this technique, however, the yield of the optical fiber preform is low.